A passive optical network (Passive Optical Network, PON for short) means that an optical distribution network (Optical Distribution Network, ODN for short) without any active electronic device situated between an optical line terminal (Optical line terminal, OLT for short) and an optical network unit (Optical Network Unit, ONU for short). Currently, typical structures include a dense wavelength division multiplexing passive optical network (Dense Wavelength Division Multiplexing PON, DWDM-PON for short) and a wavelength division multiplexing passive optical network (Wavelength Division Multiplexing PON, WDM-PON for short).
To reduce costs, a transmitter of the OLT uses a fixed-wavelength laser as a light source, and a transmitter of the ONU uses a variable-wavelength laser as a light source. Due to process errors, semiconductor lasers of a same design on a same wafer have operating wavelength differences by several nanometers. This difference causes a decreased yield of fixed-wavelength lasers for use in a PON system. Therefore, to increase the yield of such fixed-wavelength lasers, a downlink adjacent channel wavelength spacing in a PON needs to be appropriately increased. For example, the adjacent channel wavelength spacing needs to be increased from 100 GHz to 400 GHz. The ONU has higher costs when a variable-wavelength laser has a larger variable range. Therefore, to reduce the costs of a variable-wavelength laser on the side of the ONU, an uplink adjacent channel wavelength spacing in the PON needs to be appropriately reduced. Considering these problems, as compared with a system in which an uplink adjacent channel wavelength spacing and a downlink adjacent channel wavelength spacing are symmetrical, a system in which an uplink adjacent channel wavelength spacing and a downlink adjacent channel wavelength spacing are asymmetrical may achieve lower costs.
Two arrayed waveguide gratings (arrayed waveguide grating, AWG for short) having unequal wavelength spacings and N thin film filters (TFFs) are respectively used on a user side and a network side to implement the system in which an uplink adjacent channel wavelength spacing and a downlink adjacent channel wavelength spacing are asymmetrical in the prior art. A schematic diagram of the system is shown in FIG. 1. The TFFs are configured to demultiplex or multiplex optical signals in an uplink band and a downlink band. The two AWGs are respectively configured to demultiplex or multiplex optical signals in an uplink band and a downlink band in a PON system. A TFF first demultiplexes optical signals on an uplink band and a downlink band, and splits optical signals in the two bands, and the split optical signals respectively enter an AWG 1 and an AWG 2. The AWG 1 and the AWG 2 are two splitting devices, and each of the AWG 1 and the AWG 2 has a working characteristic of equal adjacent channel wavelength spacings. However, the AWG 1 and the AWG 2 work in different bands respectively, and an adjacent channel wavelength spacing of the AWG 1 is unequal to an adjacent channel wavelength spacing of the AWG 2. An optical signal obtained through splitting by the AWG 1 and an optical signal obtained through splitting by the AWG 2 paired with the AWG 1 are multiplexed using the TFFs. It can be learned that more devices are used in an existing solution of the system in which an uplink adjacent channel wavelength spacing and a downlink adjacent channel wavelength spacing are asymmetrical.